Selenite
Selenite is one of the three forms of pure natural crystalline gypsum, with one other, satin spar having a very similar features, the other natural form is gypsum rock which is commonly crushed and used in the manufacture of plaster products.
In old English gypsum was referred to as spear stone (spærstn) because of its crystal spear-like projections, of which selenite and satin spar have the most notable. Selenite crystals are much rarer than satin spar crystals, whilst gypsum rock is relatively common. Other forms of crystalline gypsum are known as tabular gypsum, which is normally shorter blocks of satin spar, whilst desert roses, form with grains of sand and gypsum flower which are referred to as crystal habits.
Selenite crystals form in large flat, transparent crystals, often called plates or windows, whilst Satin Spar crystals are fibrous and white, often appear as raw rulers or wands which can be polished into palmstones or spheres. The naturally large flat transparent form of selenite crystals, might also be called Lapis Specularis a form of selenite that was put to good use by the Romans in the construction of specularia (sometimes referred to as specularium), which included the first types of windows before the development of glass for use in glazing, it was used to construct Roman winter gardens for horticulture.
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